The invention relates to photomasks useful in the photolithographic fabrication of semiconductors. In particular, photomasks that are coated with a thin film of chemically inert poly(fluorocarbon) polymer.
The manufacture of semiconductor devices typically involves applying a layer of a photosensitive substance (a photoresist) to the surface of an target wafer. The photoresist is then exposed to light in a selected pattern using a photomask, and the photoresist is then developed to leave exposed regions of wafer. These regions are subsequently etched away or otherwise modified, and the residual photoresist is removed. The pattern of the photomask typically possesses extremely fine details, and the presence of even tiny particles on the surface of the photomask can interfere with the accurate reproduction of the pattern on the target wafer.
To minimize particulate contamination at the mask surface, optical pellicles have been developed that protect the photomask. An optical pellicle is a frame-mounted transparent membrane that is attached to the photomask surface, so that contaminating particles fall onto the pellicle membrane and not the surface of the photomask. The pellicle frame holds the pellicle membrane at a sufficient distance above the mask surface so that any particles that may fall upon the membrane lie outside the focal plane of the illuminating light, and so fail to interfere with the projected mask pattern. The use of optical pellicles in semiconductor manufacture has helped mitigate the effects of contamination by dust and other particulates, and has become widespread in the industry.
However, constant demand for smaller, faster, and more powerful microprocessors has required the semiconductor industry to fabricate smaller and smaller semiconductor circuits. Manufacturing techniques have advanced to the point that the size of the circuits being produced is effectively limited by the wavelength of light used in the photolithographic process, with shorter wavelength radiation permitting finer details in the resulting circuit structure. Thus, photolithography using 248 nm, 193 nm, and 157 nm (Deep Ultra-Violet, or DUV) illumination has become common, and even the use of 13.5 nm (Extreme Ultra-Violet, or EUV) illumination is known.
However, as the wavelength of the radiation used decreases, the energy of that light increases. Many airborne organic compounds that were benign at longer wavelengths will become photolytically activated when exposed to energetic ultra-violet illumination. For example, light with a wavelength of 248 nm reacts with most halogenated organic compounds, and may interact with some non-halogenated organic compounds. Light having a wavelength of 193 nm reacts readily with a wide range of organic airborne contaminants, and 157 nm light is efficiently absorbed by and generates reactions with even the moisture present in air. The reactive breakdown products of these reactions interact with the mask pattern, resulting in the generation of a variety of defects.
Unfortunately, the source of many of these potential contaminants is the optical pellicle itself. Volatile components are released from the pellicle membrane, from various anti-reflective coatings on the pellicle membrane, from the anodized surface of the pellicle frame, or from the adhesives used to attach the pellicle membrane to the frame or the frame to the photomask. These contaminants are essentially trapped in the space beneath the pellicle membrane, even when vents are present in the pellicle frame. Repeated exposure of these contaminants to UV illumination results in the generation of chemically reactive species, and the creation of defects on the photomask. The generation of such reactive species dramatically shortens the lifetime of the photomask, as any defects being generated could be transferred to the photoresist, with the resulting production of flawed circuit patterns. Manufacturers are faced with either frequent replacement of photomasks, or the potential generation of defective products. In either case, manufacturing costs increase and quality controls must be tightened.
The continuing necessity of protecting the photomask from particulate contamination requires that optical pellicles continue to be used. What is needed is a method of protecting the photomask and its mask pattern from the reactive contaminants that are generated during illumination that does not itself compromise the utility of the photomask, and is compatible with existing optical pellicles and manufacturing techniques. The use of the photomasks of the invention permits illumination at highly energetic UV wavelengths, while minimizing the interaction of particulate or reactive contaminants with the surface of the photomask.
Coated photomasks for photolithographic processes, where the photomask includes a transparent substrate with a pattern on the surface of the substrate defining light-transmitting and non-transmitting portions, and a protective film on the surface of the photomask comprising an amorphous poly(fluorocarbon). The invention also includes the preparation of the coated photomasks, and the use of the coated photomasks in a photolithographic process.